Water on Earth

• Water molecules have chemical characteristics that support life
• They are polar which facilitates bonding with other chemicals
• "universal solvent"
• Liquid over a wide range of temperatures
• Stabilizes systems against change
• Water ­based processes occur in a variety of conditions
• Strong cohesion
• Facilitates the transport of chemicals
• High heat capacity
• Transparent to light
• Photosynthesis is possible
• Solid less dense than liquid
• Floating ice insulates water

Earth's Early Atmosphere

• Primitive, inhospitable and very different from the one we have now
• Role of volcanic degassing
• Reducing atmosphere­ $\rightarrow$ no "free" oxygen (oxidizing atmosphere)
• Rich in CO₂, CO, SO₂, CH₄ etc.
• Hot, lots of volcanism
• Dense­, high atmospheric pressure

Dynamic Equilibrium

• The earth's atmosphere-­hydrosphere­-geosphere interacted dynamically
• Surface cooled enough to sustain liquid water
• Oceans began to accumulate (~4.4 Ga)
• Still hot
• Water precipitated and re-­evaporated
• Water + atmospheric gases $\rightarrow$ acids
• Resulted in weathering of minerals
• Cyclical chemical and physical interactions:
• Hydrosphere + atmosphere + geosphere

Proposed Hypotheses to Explain Life's Origin

Heterotrophic Hypothesis

• "Primordial Soup"
• Life originated from inorganic chemicals in the ocean

Panspermia Hypothesis

• First life forms used organic compounds for energy
• "Seeds" from space
• Microbes delivered to Earth through meteorites
• Organic compounds found in meteorites

Chemoautotrophic hypothesis

• Ocean depths
• Life originated at deep-sea hydrothermal vents
• First organisms were chemoautotrophs

Fossil Evolution

Fossils - Remains of a long-­dead organism, preserved in stone

Fossil Record - Gives info about the history of past life

• New life forms have evolved throughout Earth history

Macro-Fossils - Chemical evidence that "life was here"

• Not all fossils are macro-fossils

Central role of life in the evolution of Earth systems

• Early life forms:
• Chemo synthetic, thermophilic bacteria
• Photosynthetic cyanobacteria ("blue-­green algae")
• Life's two main influences on the chemistry of atmosphere
• Buildup of oxygen through stromatolites and aerobic which are Cyanobacteria
• Removal of carbon dioxide through early life, organic matter deposited as sediment and carbon cycle

Unique Characteristics of Earth

• Specific chemical composition of atmosphere
• Nitrogen and oxygen rich
• Existence of water in three states near surface
• Solid, liquid, vapour
• Dynamic nature of rock weathering
• Soil $\rightarrow$ organic + inorganic components
• Dynamic nature of lithosphere
• Plate tectonics

Conclusions

• Earth has several characteristics that make it unique amongst planets
• Earth has the capacity of supporting life
• Early Earth was hostile to life
• The atmosphere, hydrosphere, and geosphere have evolved chemically in dynamic equilibrium
• Life played a central role in the chemical evolution of Earth's environmental systems